1. Field of the Invention
The present invention relates to a de-icing and anti-icing arrangement for a Wind Energy Converting System (WECS), a Wind Energy Converting Systems (WECS) comprising a de-icing and anti-icing arrangement and a method for preventing and avoiding the ice accretion on the rotor blades of wind turbines in a Wind Energy Converting Systems (WECS). More particularly, what is provided is a system for preventing and/or eliminating ice accretion on the rotor blades of a WECS when the system operates in determined climatic and environmental conditions.
2. Background Information
Ice accretion on the profile of a wing, and more specifically on a wind turbine rotor blade, seriously affects its fluid-dynamic characteristics. In particular, ice accretion affects the lift and the drag of both the blade and the overall structure, and thus, remarkably changes the pressure distribution along the relevant surfaces. Often it is very difficult to foresee how factors such as lift, drag and pressure will change depending on the deposit of ice on those surfaces. In instances of ice accretion, consequently, the blade in operation undergoes different flexion and torsion stresses, which typically result in a remarkable decrease in the overall aerodynamic efficiency of the wind turbine.
The power generated by the WECS rotor when ice is present on the rotor blades is much less than that generated without icing. In addition, ice accretion also affects the blade, due to the increased and different masses distribution caused by the ice, such that it completely modifies the static and dynamic behavior of the blade.
Moreover, in ice accretion conditions a significant problem arises relating to the WECS safety, both as it relates to the safety of people or property near to the system, and to the possibility of system failure or breakdown. In fact, in situations where the turbine is in operation and the blades are functioning and have ice on their surfaces, it may happen, in a fully unforeseen manner, that ice pieces come off from the blades.
In such situations, the following are possible results:
1) objects or people around the system are hit by pieces of flying ice; and
2) suddenly and unforeseeable structural stresses are generated, essentially of an aeroelastic nature.
In the first instance, due to the dimensions of current generation WECS, where the rotors may extend to be ninety meters in diameter and the towers may reach one hundred meters in height, in conditions where ice comes off the blades in sudden and unforeseeable fashion, the pieces of ice become projectiles, taking off like bullets from the blades, and having the capacity to cause remarkable damage to the surrounding environment. The legal and other ramifications of such an occurrence are readily apparent and can easily be appreciated.
The consequences of the second situation are equally significant. When the WECS is in operation, vibrations of aeroelastic nature can generate significant structural stress both on a single blade of the system and on the system as a whole. Structural resonance phenomena may occur with the failure of one or more blades (i.e., an unforeseeable “flutter” phenomena in the pressure distribution on the blade surface may be very different from the designed one) as well as of the system as a whole (i.e., bridge breakdowns at the beginning of the twentieth century due to wind gusts are well known).
For many of the foregoing reasons, WECS operations are typically stopped when the presence of ice on the blades is detected. The duration of such stops varies depending on the seriousness of the problem. In instances where ice cannot be adequately removed by means of available de-icing devices, WECS systems are only used during a limited number of days per year, particularly when that are installed in areas where ice formation is common. In such areas, existing estimations indicate that an electric power loss of about the 20-50% is experienced vis-à-vis the amount of electric power produced annually in areas where a WECS is continuously working.
Several solutions for solving the of ice accretion on WECS blades are known in the art. These systems generally adhere to the following three operation principles:
I) utilizing a heat absorbing blade surface lining that gathers heat from the thermal sun radiation;
II) localized heating of the blade surfaces affected by the ice formation;
III) circulating heated air inside the blade body, and using internal heat conduction means to transmit heat to the outer blade surfaces affected by ice accretion.
Type I systems are only effective in presence of sun, and therefore only during the day and with climatic conditions of good sun radiation. As night time is the most critical period for ice accretion, however, these systems are the least effective at the time when they are most needed.
Type II systems generally use sheets of electrically resistive or thermo-conductive material, embedded inside the blade surface and heated for Joule effect. Such sheets are electrically heated and during the construction represent additional and electrically conductive masses applied to the shallow layers of the blade, specifically in the icing prone areas.
Type II systems prevent ice accretion during WECS operation and eliminate ice formed on the blades during a system stop. However these systems have drawbacks such that they are typically used only in research or educational applications.
More specifically, Type II systems use a complicated ice accretion control and management mechanism. This mechanism includes ice locating sensors, and control and management processing software for providing an electric power supply to the area where there is the danger of ice accretion. The complexity, cost, reliability and maintenance problems of Type II systems, consequently, are remarkable in the long run.
Moreover, the electric power necessary to heat a Type II system by means of the Joule effect may represent a significant share of the total power output of the WECS, and in specific conditions, a Type II system may even absorb as much electric power as it produces. In such cases, the actual efficiency of the system is drastically reduced, and provides a rather unsatisfactory yield in critical operating conditions.
A further drawback of the Type II system is that when idle, that is, when the rotor is moving without electric power being produced, the electric power needed to prevent or eliminate the ice accretion has to be taken by the electric grid, and the system is unprofitable in these conditions.
The thermo-conducting sheets that are glued to the blade surface of the Type II system also wear out very easily, thus they often require maintenance work and reduce the availability of these systems. The sheets are made of a substantially metallic material and can attract atmospheric lightning. Lightning can seriously damage the de-icing and anti-icing arrangement as well as the electric devices associated with the system and, in some cases, cause failure of the rotor on which the lightning discharges.
In addition, once the thermo-conducting sheets are electrically powered, they generate very strong electrostatic rotary fields, and create pollution and undesired electromagnetic noise effects around the WECS.
Another significant disadvantage of Type II systems is that any guarantee given by the manufacturer of the wind turbine is lost once a de-icing and anti-icing arrangement is incorporated onto or within in the blade structure.
Furthermore, as it has been experimentally verified, in determined environmental and climatic conditions, ice may build up anywhere on the blade surface, and thus the blade should be almost entirely covered by thermo-conducting sheets.
The consequent manufacturing and maintenance costs reach prohibitive levels, which, together with a not certainly high total efficiency of a Type II system, thus make it decidedly unprofitable.
Type III systems are devices that circulate heated air inside the blade body, apt to heat up it and also the external surface by means of thermal conduction through the material which constitutes the blade body. One example of a Type III system is provided in German Patent No DE 196 21 485.
In the German patent, a recirculation of the internal air has been adopted for each blade, controlled by a fan, and the air is heated by means of electric resistances. All the components are located in the hub of the rotor. In particular, there are two pipes, which channel the heated air in the leading section of the blade, while a pipe is apt to extract it from the rear section of the blade in order to allow the internal recirculation of the air.
The device includes small output holes on the far end of the blade, in order to avoid the collection condensed water where the stream is colder being more distant from the heat generator. However, in fact, the far end of the blade is the point where the ice accretion is more likely. In order to bring as much heat as possible to the end, a continuous support of thermally conducting material, e.g. aluminum is provided inside the blade, close to the leading edge. This creates a thermal bridge for the heating, as the rotor blade is generally made of composite material with reduced thermal conductivity, for instance resin-glass.
Such a Type III system includes at least the following drawbacks. The first is that the actual dimensions of the blades, which may reach in some points 60 mm, require a large amount of thermal energy to power the circulating air inside the blade and effectively heat up the whole blade body as well as the relevant external surface. Assuming that it is possible to attain such a level of power, the wind turbine equipped with such a system, will nonetheless still show a very low efficiency when there is the danger of icing. This because, it is necessary to heat up the whole blade mass in order to heat up the surface of the blade, and thus the amount of electric power to be transformed into heat is actually remarkable.
The use of fans inside rotating elements of such Type III systems is known to be highly discouraged by fan manufacturers as well as there is a high probability of malfunction and breakdown due to the effect of Coriolis forces on the rotating parts of the fan.
In summary, the invention of the German patent of forced air circulation may be actually carried out only when the system is stopped, with all the resulting logistic limits that are easily foreseeable.
German documents DE 842 330 and DE 198 02 574 illustrate Type III systems wherein each wind turbine blade includes an opening on an airfoil, located at the end part of the blade span, and an air stream flows into the blades and is warmed by over lapping electric current generators/motor parts.
The openings in the airfoil blade are laid out and designed to allow the warmed air stream inside the blade to continuously flowing into the blade for centrifugal effects, so exchanging heat to the blade inner parts heats the blade.
German Utility Model No DE 200 14 238 U1 illustrates a device where air is circulated inside the blade body and heated up by the waste heat given off from electric devices contained in the nacelle of the wind turbine. A ventilation system which operates with the rotor is also employed, as the fan for the forced air circulation is located in the nacelle of the arrangement. A continuous heated air distribution system is also provided inside each of the blades.
This model has the disadvantage that it is rather complicated and difficult to carry out because it uses intermediate fluids for realizing the thermal exchange between the waste heat of the electric devices and the circulating air inside the blade.
Moreover it has also the same drawback discussed with respect to the German patent, i.e. that the remarkable thickness of the blade, constituted by poor conducting materials, do not guarantee an effective and efficient heating of the surfaces of the blades.
Furthermore, the heat supplied by means of the forced circulation of the fluid stream inside the electric devices is certainly not sufficient for avoiding the problem of the ice accretion in particularly critic environmental conditions. To this we always have to add a consistent amount of heat, typically obtained by means of the Joule effect.
To be underlined is certainly the poor material thermal conductivity constituting the blade, which remarkably jeopardizes the effective heat exchange between the fluid stream and the blade body. In synthesis, even complicating the construction of the blade with the insertion of thermally conductive metallic parts, which shall reach the far end of it, the arrangement appears of poor effectiveness. The efficacy increases if a great amount of electric power is taken from the electric grid in order to heat up the blade bodies of the wind turbines, with the disadvantage of having a poor total efficiency of the WECS in critic operating conditions.
Summing up, it is indeed the type of heat transmission chosen, namely the heat internal convection from the blade body to the outside surface, which represents the main limit for the de-icing and anti-icing effectiveness. This method of heating, in fact, leads to the use of large amounts of thermal power for the desired purpose, and to not being able to send heat only to the particular surface areas of the blade concerned by ice. More specifically, the tip area of the blade, which is the most affected by ice accretion, is the exact area where the internal airflow arrives with the lowest temperature, having already expended its heat to the areas closest to the root of the blade.